In an effort to comply with upcoming Federal Communications Commission broadcast requirements, television stations across the United States are adding digital broadcasting capability while maintaining present analog broadcasting capability. The changeover from analog to digital is motivated at least in part by FCC requirements mandating that analog television broadcasts be phased out and replaced over time by all digital television broadcasts. The amount of time a station has to begin and comply with the changeover is dependent on a number of factors including the size of the television viewing market served by the station. Stations adding digital broadcast capability cannot simply reuse existing equipment in many instances due to more restrictive digital broadcasting parameters.
Broadcasting digital television signals, particularly digital UHF television signals, involves more stringent parameters than those involved with analog signal broadcasting. This is especially true with regard to the degree of frequency cutoff sharpness required at the upper and lower frequencies passed through a bandpass filter. The number of cavities in a bandpass filter are a factor in determining the sharpness of the frequency cutoff. A bandpass filter with several cavities will have a sharper cutoff at the upper and lower frequencies than a bandpass filter with fewer cavities. While a multiple cavity bandpass filter comprises a single component required for digital broadcasting, it can be a very expensive component.
Bandpass filters are very expensive due to current filter manufacturing methods and filter tuning methods. Coupling plates comprise a significant portion of the cost of a bandpass filter. Several coupling plates are used in a single cavity bandpass filter. Specifically, coupling plates are used to attach adjoining components to a filter such as a waveguide transmission feed line. Additionally, coupling plates are also used to attach multiple cavities of a filter together in the case of a multiple cavity bandpass filter. The coupling plate itself comprises an iris defining an aperture. The tuning of a bandpass filter is dependent upon the size of the iris in the coupling plate.
A bandpass filter must be tuned and adjusted prior to use for a given application. Current tuning methods involve individually tuning each iris coupling plate used in a filter by adjusting the size of the iris. Current manufacturing and assembly methods employ an iterative process that involves assembling the components of a filter, measuring the characteristics of the assembled filter, disassembling the components of the filter, adjusting the filter tuning by machining out the coupling plate iris, assembling the components of the filter and repeating the process. By virtue of the iterative tuning process, each coupling plate within the same filter has a unique iris aperture size thus limiting use of each coupling plate to a specific position in a specific filter configuration.
In addition to the long standing prior art coupling plate manufacturing problem requiring customization by individual machining of every iris coupling plate, prior art coupling plate tuning methods are also subject to a waste problem. The opening of an iris can only be made larger using prior art tuning and manufacturing methods involving machining out the iris. If an iris opening is made too large by machining it out, then another plate must be made, thus causing waste of material, as well as repeated effort to start the tuning process over again.
Another tuning method currently used involves adjusting the tuning of a filter using metal tuning components. The metal components are adjusted and then held in a fixed position with threads or a lockable sliding mechanism. Using metal tuning components, however, has several drawbacks.
Contact problems can be the biggest problem with using metal tuning components, and in some instances are destructive and quite costly. One type of contact problem is brought about by insufficient contact between tuning components and the filter. Insufficient contact can permit too much movement between tuning components and the filter. Another type of contact problem is brought about when the degree of contact between the filter and the tuning component does not permit enough or any movement between components and the filter.
Initially a contact problem causes local heating at the contacts. The accelerated heating in the apparatus may change the apparent tuning. The increased heat speeds up the rate of oxidation of metal components. Oxidation of metal components further exacerbates the initial contact problem and continues the problem cycle which will ultimately lead to the destruction of the contacts. If a filter using metal tuning components is employed in a high power application, such as digital UHF broadcasting, a slight variation in the apparent filter tuning may result in the buildup of enough heat to damage the filter.
Contact problems, in addition to adversely effecting the digital broadcast of the station in question, may interfere with other digital broadcasts. Due to stringent parameters involved with digital broadcasting, changes to the apparent tuning of a filter brought about by contact problems may interfere with the digital broadcast of neighboring television stations serving the same broadcasting market. Causing interference with other stations by broadcasting outside of the frequencies specified in a television station's broadcasting license may have adverse consequences with the FCC.
One approach to control problems is the use of silver plating to minimize corrosion and facilitate electrical conductivity. However, even with silver plating, metal tuning components may be susceptible to contact problems.
Thus, there is a need in the art for a novel coupling plate that is cost effective and can ultimately reduce the cost of bandpass filters required for applications such as digital television broadcasting. There is also a need in the art for a novel coupling plate that alleviates the contact problems associated with using metal tuning components. There is also a need for a novel coupling plate that is tunable without undesirable contact problems, and that does not require adjusting the size of the iris by machining it out.